1. Field of the Invention
The present invention relates to a fuel injector used in, for example, a gas turbine engine and including a combined fuel injector configured by combining a plurality of fuel injectors, and particularly to a pilot injector.
2. Description of the Related Art
In recent years, in consideration of the environment, there is a need for a reduction of NOx (nitrogen oxide) emitted from gas turbine engines. The NOx to be emitted from the gas turbine engine is generated mainly by oxidization of nitrogen in inflow air when fuel is supplied to the inflow air and combusted at high temperature. Meanwhile, the amount of CO2 emission of the gas turbine engine, that is, fuel consumption decreases as an exhaust gas at an exit of a combustor increases in temperature. Therefore, to reduce the CO2, the fuel needs to be combusted at high temperature by increasing a fuel-air ratio. According to a fuel nozzle of a combustor of a conventional gas turbine engine, the fuel is directly sprayed to a combustion chamber without premixing the fuel with the air. Therefore, before the fuel is adequately mixed with the air, the fuel combusts, and regions where a flame temperature is significantly higher than an average value are generated locally. The amount of NOx generation increases exponentially with the flame temperature. Therefore, a large amount of NOx is generated from the local regions where the flame temperature is high. On this account, according to the conventional combustion method, when the temperature of the exhaust gas at the exit of the combustor is increased, the amount of NOx emission increases sharply.
To reduce the local regions where the flame temperature is high, a lean premix combustion method is effective. According to this method, the fuel and the air are premixed, and a fuel-air mixture in which the fuel in the form of a mist is dispersed in the air is supplied to the combustion chamber and combusted therein. Meanwhile, according to the lean premix combustion method, in a case where the output of the gas turbine engine is low and the fuel-air ratio is low, the flame is unstable and incomplete combustion tends to occur as compared to a case where the fuel is directly sprayed to the combustion chamber. Here, a concentric fuel injector has been devised. This fuel injector is configured such that a pilot injector and a main injector provided outside the pilot injector are provided coaxially. When the output of the gas turbine engine is low, the fuel is directly sprayed from only the pilot injector to the combustion chamber to maintain stable combustion. When the output of the gas turbine engine is intermediate or high, that is, when the amount of NOx emission is large, the amount of fuel injected directly from the pilot injector is reduced, and a pre-mixture generated by the main injector is also injected to the combustion chamber. With this, the amount of NOx emission is reduced. Regarding a gas turbine engine for aircrafts, the output of the gas turbine engine is substantially low (lower than about 40% of the rated output) in a state of each of ground idle, flight idle, and approach, the output of the gas turbine engine is substantially intermediate (about 40 to 80% of the rated output) in a cruising state, and the output of the gas turbine engine is substantially high (about 80 to 100% of the rated output) in a state of each of climb and takeoff.
According to the concentric fuel injector, when the output of the gas turbine engine is low, that is, when only the pilot injector is operating, the air flow not containing the fuel flows from the main injector into the combustion chamber. Therefore, the pilot fuel in the form of a mist may interfere with the air flow injected from the main injector, and this may deteriorate the combustion efficiency, ignitability, and flame holding performance. To avoid this, a fuel injector has been proposed, in which: a pilot combustion region and a main combustion region are largely separated from each other to prevent the pilot fuel in the form of a mist from interfering with the air flow injected from the main injector (see Japanese Laid-Open Patent Application Publication No. 2007-162998).
When the output of the gas turbine engine is intermediate, that is, when the output of the gas turbine engine is gradually increased from the low output and the supply of the pre-mixture from the main injector is started in addition to the fuel injection from the pilot injector, the temperature of the air flowing into the combustor is not yet adequately high. Therefore, to achieve stable combustion of the main pre-mixture, a flame holding effect by the pilot flame with respect to the main pre-mixture is important. According to the fuel injector of Japanese Laid-Open Patent Application Publication No. 2007-162998, the pilot combustion region and the main combustion region are largely spaced apart from each other. Therefore, when the output of the gas turbine engine is intermediate as above, the flame holding effect by the pilot flame with respect to the main pre-mixture is small, and the combustion efficiency of the main injector lowers. On this account, the fuel can be supplied to the main injector only when the output of the gas turbine engine is adequately increased, the temperature of the air flowing into the combustor is high, and the combustion stabilizes only by the main pre-mixture. When the output of the gas turbine engine is less than the above, only the pilot injector is used. Therefore, when the pilot combustion region and the main combustion region are largely spaced apart from each other and the flame holding effect by the pilot flame with respect to the main pre-mixture is small, a gas turbine engine operation range in which the NOx reduction can be realized by using the premix combustion of the main injector narrows.